Another problem with priority boarding, is that in order for it to be strictly ahered to, the agent must check every boarding group or sequence number religiously. This is a possible distraction from the more important things that should be checked on a boarding card and in my opinion is another task to slow things down.
If you allocate seats and ask for passengers in rows 12-30, ok maybe only 70% or so will get it right, but it's not something that needs to be checked. If passengers seated at the front slip through, it's not the end of the world. However, if you let someone down in the wrong boarding group, you're likely to start a riot!
Bit of a thread creep here, but has anyone ever boarded a flight with a gate reader set to comfort boarding. How accurate is this at checking seat numbers?